Epizootic Investigation And Aetiological Study On The Bacterial Diseases In Cultured Turbot (Scophthalmus Maximus)

Turbpt, Scophthalmus maximus (L.) was introduced from the U.K. to China in 1992. Since then, turbot had been extensively cultured and becoming an important industry along the coast of north China. With the rapid development and intensive culture, however, many kinds of diseases occurred frequently, causing massive mortalities and economic losses. In recent years, our investigation showed that the bacterial diseases were the most common diseases in cultured turbot in China. Therefore, etiological study was conducted in order to solve the disease problems and maintain the sustainable development for the industry.Base on the diseased samples and cases derived from the coastal regions of north China, epizootic investigation has been performed during 2002 to 2003. The results shown that the bacterial diseases had high prevalence and caused severe threat to the cultured turbot, while viral and parasitic diseases were usually seldom. Fistopathological study also indicated that bacterial infections could cause severe changes among many tissues and organs.Fin rot, white feces and ascites syndromes were the common diseases in the years from 2002 to 2003, thus the aetiological study for these bacterial diseases had been emphasized. Three bacterial strains were isolated from ulcerative fins of cultured turbot derived from three different farms, they were dominant for each case, but they were proved as the same species. Through the physiological and biochemical tests and 16S rRNA sequencing analysis, the bacteria corresponding to fin rot were identified as Vibrio anguillarum. In challenge experiment, the affected fish showed clinical signs similar to naturally infected turbot, i.e., severe external hemorrhaging in fins. The fact of succeed artificial infectiondemonstrated that Vibrio anguillarum was the pathogen causing fin rot disease, and its LD50 was 1.7l03 CFU/ml.Three samples were taken from each syndrome, white feces and ascites, from which six strains being dominant were totally isolated. Based on the traditional physiological and biochemical tests and 16S rRNA sequencing analysis, two strains had been identified as Vibrio carchariae and Vibrio alginolyticus, another two strains were the same as Vibrio scophthalmi, while others were unable to be classified. Additionally, according to the examination through the smear microscopy and histopathology, bacteria were always dominance in the lesions, but none with fungal, parasitic and viral signs. We assumed that they were probably pathogens associated with the diseases. As such, the further deep study is needed to confirm their relations. Finally, drug sensitivity tests for these bacteria were carried out, it was found these bacteria were usually resistant to the antibiotics which were commonly used for long in aquaculture.